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I think it's not about the weakness of the Castle, or the heroes, but only about how to manage all of this... Perhaps influenced by Alex's games, but lately I've been enjoying playing with the Tower, and it's not going so badly. And that's with my level of play... what is a good player capable of?
In my time, I've played many games with the Tower (on TE), including serious tournament games, and had series of 7-8 games in a row with this Castle.
Conclusion - success when playing with the Tower depends on many small things. Often, the game breaks because of some trivial thing. There are many nuances that you need to watch and pay close attention to.
For example, whether or not a Scholar comes (if it's not the starting unit), whether there are heroes with starting Haste, and generally, pay close attention to what spells come on level 111 (with a starting Scholar). You also need to carefully look for opportunities for a second front (mages + nagas) and timely train the necessary spells for them.
There can also be problems with resources; nagas are usually built, but there might not be enough gold for an upgrade, especially if you put the 2nd guild on the Companion for expanding the magical knowledge.
You also need to earn money more carefully; you need a lot of it and quickly, so the 2nd and 3rd fronts are welcome. In general, you need to play much more carefully than with many other Castles. The planning horizon should be a day or two longer.
I've already written about battles. You can't win many battles with a cavalry rush; you need to prepare for many of them in advance. By the end of the 2nd week, Gargoyles, Golems, and Nagas are often upgraded, but the upgrade sequence is very variable... it depends not only on the money but also on the planned battles. Plus, sometimes you have to build and upgrade Djinns on the second front (against some snipers).
Plus, heroes often run out of mana, so you have to fight battles with different heroes and look for wells. By the way, this is one of the reasons for the popularity of the starting Josie - Haste in the book and Mysticism from the start.